1. Field of the Invention
The present invention relates to an engine controller that controls an air-fuel ratio, ignition timing, etc., and specifically relates to an engine controller capable of enhancing exhaust emission characteristics at the time of start-up.
2. Background Art
In recent years, with the tightening of regulations on automotive engine exhaust around the world including North America, Europe and Japan, there is a demand for further enhancing the exhaust emission characteristics of engines. In order to enhance the catalyst performance and the accuracy of catalyst control to conform the latest or future exhaust regulations, the main problem to be solved is reduction of the amount of HC (hydrocarbon) emitted for several tens of seconds after start-up of the engine and before activation of the catalyst.
It is known that retarding the ignition timing is effective for reduction of HC (emission amount) before catalyst activation. In particular, when the ignition timing is retarded, the thermal efficiency is lowered, and thus, in order to maintain a constant torque, it is necessary to increase the fuel amount (air amount), and accompanied by that, the amount of generated heat is also increased. As a result of the increase in the amount of generated heat, the temperature of the exhaust is raised, and unburned fuel is burned in combustion chambers and an exhaust pipe, substantially lowering the HC concentration. Furthermore, as a result of the exhaust temperature increase, catalyst activation is advanced.
As illustrated in FIG. 19, an HC reduction effect provided by retarding the ignition timing varies depending on the air-fuel ratio. In particular, it can be seen that a largest HC reduction effect can be obtained with an air-fuel ratio of from 15 to 16. Meanwhile, it has been known that the air-fuel ratio is made to be lean, the stability of combustion deteriorates. Furthermore, it has been known that if the amount of retarding is increased, the stability of combustion also deteriorates. In the environment of practical use, various disturbances, such as variations in fuel quality and manufacturing errors and temporal changes of various types of devices, occur, and thus, it is necessary to ensure robustness against these disturbances, that is, it is necessary to secure a margin for stability as well as the air-fuel ratio and the retarding amount, making it impossible to set the air-fuel ratio and the retarding amount to be in a condition in which the HC concentration is the smallest, as illustrated in FIG. 20.
From the circumstances described above, as can be seen in, e.g., JP Patent Publication (Kokai) No. 2007-303354 A, many methods in which on-board optimization of the air-fuel ratio is performed in a very early period of the start-up time using a parameter correlated with the air-fuel ratio, such as engine speed variation, has been proposed. Also, as can be seen in JP Patent Publication (Kokai) No. 8-122099 A (1996) (JP Patent No. 3574853), many methods in which on-board optimization of the ignition timing (retarding amount) is performed also using a parameter correlated with the combustion stability, such as engine speed variation, has been proposed.